In one type of commonly used electrical motors, a stator comprises windings in which an electrical field creates a rotating magnetic field. Inside, or circumferentially outside the stator, a rotor is rotationally attached to rotate under influence of the magnetic field. Various principles exist. In a synchronous motor, the rotor is magnetised, or comprises a set of permanent magnets. This type of motor is simple and reliable, and the rotational speed of the rotor corresponds to the rotational speed of the electrical field in the windings of the stator. In certain applications, however, the synchronous motor has an inappropriate start-up characteristic. In asynchronous motors, the rotor comprises substantially longitudinally extending windings which in axially opposite ends of the rotor are interconnected by short circuit rings. Typically, a rotor for an asynchronous motor comprises a rotor core made from a magnetically conductive material and a squirrel cage wherein the windings and short circuit rings are moulded in one piece from an electrically conductive material, e.g. aluminium. The rotor could be laminated from sheets of a metal, wherein each sheet comprises an opening which, in combination with other sheets, forms a conductor slot extending axially throughout the rotor. After the assembly of the sheets into a rotor core, conductive bars, constituting the windings, are moulded directly into the conductor slots using the slots as a mould, and the short circuit rings are moulded as an integral part of the bars. In use, an electrical current is induced into the windings of the rotor by the magnetic field generated in the stator, and due to a shift between the electrical field in the windings of the stator and in the windings of the rotor, the rotor starts to rotate. Such motors have good start-up characteristics but in order to continue the induction of an electrical field into the windings of the rotor, the electrical field of the stator must move relative to the windings of the rotor. The rotational speed of the rotor will therefore always be lower than the rotational speed of the electrical field in the stator. To increase the speed of the rotor, a rotor for a line-start motor comprises, in addition to the windings, a set of permanent magnets, and a line-start motor thereby combines the advantages of synchronous and asynchronous motors.
Since large temperatures during the moulding of the squirrel cage may influence the strength of the permanent magnets or even destroy the magnets, the magnets are normally inserted into the cavities after the squirrel cage has been moulded. In one known motor of this kind, the magnet cavities are formed by openings, typically quadrangular openings, extending through the rotor from one axial end face to the opposite axial end face. In one of the end faces, the opening is closed, or the opening area is reduced by a first end plate having either no opening or an opening of a reduced size. The end plate forms part of the rotor stack and it is typically made from the same material, i.e. a magnetically conductive material. In an opposite end, the magnets are inserted into the cavities, and a second end plate is subsequently fitted to the opposite end face to secure the magnets. Analogously to the first end plate, the second end plate of the known motors has either no openings, or openings of a reduced size.